Effect of Hydrostatic Pressure on Ionic Conductivity in Doped Single Crystals of Sodium Chloride, Potassium Chloride, and Rubidium Chloride

The effect of hydrostatic pressure up to 9000 kg/${\mathrm{cm}}^2$ on ionic conductivity in NaCl, KCl, and RbCl single crystals doped with divalent impurities has been studied over the temperature range 200° to 500°C. The conductivity in this temperature range is due almost entirely to the motion of extrinsic cation vacancies. The activation volume $\Delta V_m$ for motion of the cation vacancies is 7.7±0.5 cc/mole in NaCl doped with Ca${\mathrm{Cl}}_2$ and 7.0±0.5 cc/mole in KCl doped with Sr${\mathrm{Cl}}_2$. The results are in fair agreement with values predicted on the basis of Keyes's empirical expression relating activation volume to activation energy and isothermal compressibility. Sample materials were chosen with the view of testing for a correlation between activation energy and shear modulus. The small shear modulus in KCl and RbCl decreases with increasing pressure, while the reverse is true for NaCl. However, the data are not adequate to draw definite conclusions about such a correlation. The conductivity of RbCl doped with Ba${\mathrm{Cl}}_2$ increases by an order of magnitude at the phase transition from the NaCl to the CsCl structure. At 300°C, the transition occurs at 6100 kg/${\mathrm{cm}}^2$.